The reason it is not implemented is because these functions cannot be
defined for a general MonadIO. However, these functions can be easily
defined for a large and interesting subset of MonadIO. The following
code demonstrates that. It uses no extensions (other than those needed
for the Monad Transformer Library itself), patches no compilers, and
proposes no extensions. The generic catch has been useful in a
database library (Takusen), where many operations work in a monad
(ReaderT Session IO): IO with the environment containing the database
session data. Many other foreign libraries have a pattern of passing
around various handles, which are better hidden in a monad. Still, we
should be able to handle IO errors and user exceptions that arise in
these computations.

The following instances presume that an exception that occurs in
m discard the state accumulated since the beginning of ms execution.
If that is not desired -- don't use StateT. Rather, allocate
IORef and carry that _immutable_ value in a ReaderT. The accumulated
state will thus persist. One can always use IORefs within
any MonadIO.